It has previously been proposed to provide anti-skid systems in which a pump supplies hydraulic fluid to form a pressure source for a closed braking circuit. Valves are included in the braking circuits to, respectively, control admission, maintenance of pressurized brake fluid, or drainage of pressurized brake fluid in accordance with operating requirements of the vehicle. Such systems may include a supply valve which supplies pressurized brake fluid to the actual control valve which varies the braking fluid pressure at the brake, the supply valve insuring that, at all times, sufficient pressurized brake fluid is available for the actual hydraulic brake cylinder-piston arrangement at the respective brakes. It has already been proposed to monitor the pressure within the braking fluid pressure supply which, if the pressure should drop below a predetermined level, issues a warning signal and/or a switching signal which controls supply of the pressurized brake fluid to the supply valve and, rather, permits brake actuation under control from the master cylinder of the braking system.
Braking systems of the above-mentioned type are discussed in an SAE ("Society of Automotive Engineers") report, which describes the system including a warning switch constructed as an opening-type switch which provides an immediate indication if a signal line is interrupted since interruption of the signal line will also simulate opening of the switch. Such opening of the switch or interruption of the signal line is indicated, representing either electrical malfunction or failure of the pressure which is being monitored to drop below a predetermined level.